Paper electrostatic masters for lithographic printing, and the methods for making the same, are well known. Lithography depends upon the immiscibility of a greasy lithographic printing ink and an aqueous etch or lithographic solution. In use, a paper lithographic master is first imaged in a known manner, and the imaged plate is then placed on a plate cylinder of an off-set duplicating press. The overall surface of the plate is treated with an aqueous wet-out or fountain solution which wets all portions of the plate except those areas which have been imaged and are water-repellant. The press inking rolls then pass over the surface of the plate and deposit a film of ink only upon the ink-receptive image areas. In the printing operation in ink from the imaged areas is transferred in reverse to a rubber off-set blanket which in turn prints directly onto a paper sheet so as to form a copy.
Although imaging of the master can be obtained in a number of ways, the present invention is concerned with the preparation of masters suitable for imaging by photoelectrostatic reproduction. This type of reproduction depends upon the presence of a light sensitive photoconductive pigment dispersed in an insulating matrix of a resinous, film-forming material. An electrostatic charge is applied to the surface of the photoelectrostatic coating in the absence of light, and on exposure of the charged surface to an optical image, the charge is dispersed except in those areas which are imaged. Toning of the surface then converts the electrostatic image to a permanent visible image which is ink receptive.
In order to obtain a satisfactory dispersal of charge in non-imaged areas, it has been found necessary to provide under the photoconductive coat an electroconductive sub-coating through which the charge dispersal occurs. Many ways have been proposed to obtain this conductivity, for instance through the use of inorganic salts, humectants, quaternary ammonium compounds and electroconductive polymers.
In the case of lithographic plates, if the aqueous fountain solution works into the body of the paper, the surface of the plate may become less completely wetted-out by water because water has been withdrawn from the surface into the plate. Therefore, the surface may not repel the printing ink, and areas of the surface which should be perfectly blank will darken or "tone".
In addition, absorption of water into the base paper is likely to cause fiber swelling and dimensional expansion in a cross-machine direction, in turn causing buckle or what is known as cockle of the master. As the master enters various nips on the printing press, the cockle is flattened creating a crease. This crease then picks up ink which reproduces on copies causing a streak.
These and other difficulties are particularly great when the plates are used for long runs and are therefore subjected to repeated wettings and inkings. Attempts have been made to overcome these difficulties by employing water-resistant barrier coatings beneath the photoconductive layer. A typical barrier coat may contain on a weight basis about 15% styrene-butadiene latex, about 5% casein or protein and about 80% filler, primarily coating clay. However, these barrier coatings, particularly when applied from highly concentrated solutions, are not compatible for use with conventional conductive agents. For instance, conductive salts tend to precipitate the latices in solution. Conductive polymers are cationic, and are incompatible with the anionic latices. In addition, the latices are very dielectric and coats containing such latices, when applied in the coat weights necessary to achieve adequate water hold-out, are insufficiently conductive.
In copending application Ser. No. 930,329 (now abandoned) assigned to assignees of the present application, filed by Michael J. Shaw et al on Aug. 2, 1978, there is disclosed an electrostatic paper master comprising a paper base, a barrier coat, and a photoconductive layer, the barrier coat comprising a binding amount of a binder resin and a filler at least a portion of which is an amount of plastic particles sensitive to the solvent used in the application of the photoconductive layer. The plastic particles are in an at least partially coalesced state in said barrier coat. A binding amount preferably is about 10 to about 40% binder based on the total weight of the filler. Preferably the plastic particles are selected from the group consisting of polystyrene, polyvinyl acetate and copolymers thereof, polyvinyl butyral and copolymers thereof and polyacrylate and copolymers thereof. The disclosure of said prior patent application is incorporated by reference herein.